This disclosure relates generally to refrigerant vapor compression systems and, more particularly, to improving the energy efficiency and/or low cooling capacity operation of a transcritical refrigerant vapor compression system.
Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage area in commercial establishments. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodally.
Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature. The desired temperature at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved. The refrigerant vapor compression system must not only have sufficient capacity to rapidly pull down the temperature of product loaded into the cargo space at ambient temperature, but also should operate energy efficiently over the entire load range, including at low cooling capacity when maintaining a stable product temperature during transport.
A typical refrigerant vapor compression system includes a compression device, a refrigerant heat rejection heat exchanger, a refrigerant heat absorption heat exchanger, and an expansion device disposed upstream, with respect to refrigerant flow, of the refrigerant heat absorption heat exchanger and downstream of the refrigerant heat rejection heat exchanger. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles. It is also known practice to incorporate an economizer into the refrigerant circuit for increasing the capacity and energy efficiency of the refrigerant vapor compression system. For example, a refrigerant-to-refrigerant heat exchanger or a flash tank may be incorporated into the refrigerant circuit as an economizer. The economizer circuit includes a vapor injection line for conveying refrigerant vapor from the economizer into an intermediate pressure stage of the compression process.
Traditionally, most of these refrigerant vapor compression systems have been operated at subcritical refrigerant pressures. Refrigerant vapor compression systems operating in the subcritical range are commonly charged with fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R410A, R404A and R407C. However, greater interest is being shown in “natural” refrigerants, such as carbon dioxide, for use in refrigeration systems instead of HFC refrigerants. Because carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime.
In refrigerant vapor compression systems operating in a subcritical cycle, both the refrigerant heat rejection heat exchanger, which functions in a subcritical cycle as a condenser, and the refrigerant heat absorption heat exchanger, which functions as an evaporator, operate at refrigerant temperatures and pressures below the refrigerant's critical point. However, in refrigerant vapor compression systems operating in a transcritical cycle, the refrigerant heat rejection heat exchanger operates at a refrigerant temperature and pressure in excess of the refrigerant's critical point, i.e. in the supercritical range, while the refrigerant heat absorption heat exchanger, i.e. the evaporator, operates at a refrigerant temperature and pressure below the refrigerant's critical point, that is in the subcritical range.
Operating at refrigerant pressure and refrigerant temperature in excess of the refrigerant's critical point, the refrigerant heat rejection heat exchanger functions as a gas cooler rather than as a condenser. Thus, the refrigerant leaving the refrigerant heat rejection heat exchanger is still in the vapor (gas) state and at a supercritical pressure. The higher refrigerant pressures associated with operation in a transcritical refrigeration cycle, such as in refrigerant vapor compression systems using carbon dioxide as the refrigerant, complicates operation of the refrigerant vapor compression system. Controlling the high-side pressure, that is the compressor discharge pressure, is not only more difficult, but also more important. For example, a 100 psi departure in compressor discharge pressure from the optimum set point discharge pressure for energy efficiency will result in a significant reduction in energy efficiency. U.S. patent application Ser. No. 13/121,824, assigned to the common assignee as this application is subject to assignment, and filed as the national stage application of International Patent Application No. PCT/2009/058543, filed 28 Sep. 2009, and published as International Publication No. WO 2010/039630, on 8 Apr. 2010, discloses a high-side pressure control for a transcritical refrigeration system, the entire disclosure of which is incorporated herein by reference.